Present day imprinting mechanisms, i.e. electrophotographic copiers, laser printers, etc. are generally capable of handling more than one size of media sheet (i.e., paper). Ordinarily, such mechanisms do not automatically adjust the imprinted image on the different size papers unless instructed to do so by a user input. As the market place has become more sophisticated, a need has arisen for the imprinting apparatus to adjust the image in accordance with the positioning (or mispositioning) of the paper sheet. For instance, where pre-printed forms are employed, a paper position determining system allows the imprinting apparatus to place an image accurately relative to the printed areas of the form.
In the printing of color images, where multiple passes are required with intermediate fusings, it is vital that over-printed images be in accurate registration. However, each fusing may cause a shrinkage and/or misregistration of the paper. An accurate position sensing system can be used to accurately read the paper position and size and to allow the next color to be placed on the paper accurately relative to the first color. Such a system is described in copending U.S. patent Ser. 07/561,831, now U.S. Pat. No. 5,093,674, issue Mar. 3, 1992, entitled "Method and System for Compensating for Paper Shrinkage and Misalignment in Electrophotographic Color Printing", by Storlie and assigned to the same Assignee as this application.
Storlie describes a color laser printer wherein a paper position sensor is placed in the paper path and includes a pair of optical sensors that are positioned beneath opposing edges of a paper sheet as it passes along the paper path. Each optical sensor is operative to generate an output voltage whose level varies in accordance with the amount of sensor area that is shadowed by a passing paper sheet. The voltages generated from the sensors are processed to determine the orientation of the paper sheet and signals are derived to control the print mechanism to alter the image's position in accordance with the determined paper position.
In U.S. Pat. No. 4,799,084 to Koike et al., two systems for paper sensing are disclosed. The first is used for leading-edge detection and includes a mechanical arm which is caused to rotate and block a light beam when the leading edge of a paper sheet impacts upon it. The second is used to monitor lateral positional changes of a paper sheet. This is accomplished by having the paper sheet pass over a slot through which a light beam is directed so that the paper blocks a portion of the light from reaching an oppositely disposed sensor. By measuring the sensor voltage level, the position of the paper can be found.
A problem with systems such as that shown by Koike et al. is that any changes in the light level reaching the sensor appears as a change in the paper width or position. For example, if the light level output changes or if paper dust covers the sensor, the sensor perceives these light alterations as a change in paper width.
In U.S. Pat. No. 4,839,699 to Hosaka, et al., a system for determining the image position on a paper sheet is described (broadly) and employs a photo-diode array and fiber array for determining the position of a line or other image on the paper.
As indicated above, systems which rely upon analog level potentials to determine paper position are subject to various anomalies that affect the sensor's determination. Furthermore, such systems require careful adjustment to enable accurate positional determination of paper in the paper path.
Accordingly, it is an object of this invention to provide a media-sheet position determining system that is highly accurate.
It is still another object of this invention to provide a media-image position determining system which does not rely upon an analog voltage level for its determination.
It is yet another object of this invention to provide an improved media-sheet position determining system that employ a light source already present in the apparatus.